The present invention relates to a control system for a lockup clutch.
One of the prior art control systems for a lockup clutch is disclosed, for example, in JP 63-172058. This control system for a lockup clutch is so constructed that an operating state of a lockup clutch is controlled by a solenoid having a duty ratio controlled. In order to prevent the engagement time of the lockup clutch from varying by oil temperature, vehicular speed, engine load, etc., the hydraulic pressure characteristic upon engagement of the lockup clutch is changed in response to an oil temperature signal, a vehicular speed signal, an engine load signal, etc. By way of example, due to the fact that the engagement time of the lockup clutch upon large engine load is longer than that upon small engine load, the hydraulic pressure characteristic upon engagement is changed quickly to prevent variation in the engagement time.
As to such a prior art control system for a lockup clutch, however, even if the hydraulic pressure characteristic upon engagement is adequately determined in accordance with oil temperature, vehicular speed, engine load, etc., the time required for engagement of the lockup clutch may vary due to a secular change of a friction characteristic of a clutch facing, a friction coefficient of hydraulic fluid, etc. Moreover, due to dispersion of engine output torque, etc., among units, it is impossible to unify the engagement characteristic of the lockup clutch all transmissions.
It is, therefore, an object of the present invention to provide a control system for a lockup clutch which is capable of making constant the engagement characteristic of a lockup clutch even with a secular change of friction characteristic of a clutch facing, friction coefficient of hydraulic fluid, etc.